This invention relates to the preparation of advanced ceramic materials and products, particularly to lightweight advanced ceramics having high compressive strength and high hardness. More particularly the invention is directed to carbosilane precursor materials which can be conveniently used to form silicon carbide coatings by vapor deposition at high yields and high purity. The invention includes novel chlorine-free ceramic precursor compositions and methods for making the precursor and vapor phase deposition of silicon carbide coatings. The ceramic precursor compositions can optionally include non reactive constituents such as diluents and carrier gas components. Specifically this invention is directed to 2,4,6-trimethyl-2,4,6-trisilaheptane, the preparation thereof,and the use thereof as a silicon carbide precursor in methods for thermal vapor deposition of silicon carbide on a variety of substrate materials and shapes.
Silicon carbide is a ceramic material which is recognized as useful in a wide variety of applications such as electronics, engine components, low friction bearings, thermal and environmental barrier coatings, wear resistance parts such as brakes and other applications in which high strength, thermal stability, oxidation and corrosion resistance, and low density are required.
Silicon carbide is difficult to process by conventional forming means such as sintering, machining, and spinning. Production of thin films, fibers, composites, and complex shapes of silicon carbide are particularly difficult. Silicon carbide coatings provide a hard, inert surface on a variety of substrate materials and shapes which can be regular, irregular, or complex in geometry.
Carbosilane polymers are known as precursors to silicon carbide ceramics. Illustrative silicon carbide precursors are described in U.S. Pat. No. 5,153,295. These polymers are often referred to as pre-ceramic polymers. To form silicon carbide by chemical vapor deposition, CVD, the precursor composition must be cleanly and easily vaporizable. Many polymers are viscous oils or solids, which can not be easily evaporated even under extreme vacuum. Another common precursor material is methyltrichlorosilane. However the use of this material requires a source of hydrogen gas to combine with the chlorine atoms liberated during decomposition. The reaction forms hydrogen chloride as a by-product which must be removed, thus requiring a scrubber as part of the equipment. Since the hydrogen chloride is corrosive all equipment must be corrosion resistant. Currently available chlorine-free carbosilanes are difficult and costly to produce and often contain excessive amounts of carbon relative to silicon.
Silicon carbide has special utility as a coating material for a wide variety of substrates forms and materials including solid surfaces, two and three dimensional fiber preforms, yarns, felts, woven materials, tube bores, and pre-shaped parts. Coatings can be applied to the substrate by various techniques in which a silicon carbide precursor composition is first applied to the substrate by means such as painting, spraying, and liquid infiltration. The precursor composition is then cured, if necessary, and then pyrolyzed to form the silicon carbide coating. Chemical vapor infiltration and chemical vapor deposition can be used to form coatings of varying thickness from low molecular weight vaporizable precursors in a single step or in multiple incremental steps.
The silicon carbide precursor of this invention is a liquid single chemical compound for deposition of silicon carbide on a variety of substrates and for vapor infiltration of powders, powder compacts, and fiber preforms.
The silicon carbide precursor material of this invention has the following empirical formula C7Si3H22, named as 2,4,6-trimethyl-2,4,6-trisilaheptane having the following structure: 
For convenience, the compound which is one of the aspects of this invention may be referred to as TMTSH. This silicon carbide precursor compound may be referred to as SP 2000.
This silicon carbide precursor is a chlorine-free, single compound rather than a polymer, oligomer, or mixture of compounds, oligomers, or reaction products. The main chain of the compound comprises repeating Sixe2x80x94C units. The carbon to silicon ratio in the precursor compound is a relatively low 7:3. This chlorine-free carbosilane contains no elements other than silicon, carbon, and hydrogen and is therefore highly suitable for chemical vapor deposition and chemical vapor infiltration applications. TMTSH provides higher deposition rate and higher yield than can be achieved with methyl-trichlorosilane. Other benefits include ease of preparation, handling, storage, and transportation. The composition is noncorrosive.
The material can be applied to a wide variety of substrate materials and substrate preforms by deposition and infiltration in vapor form to provide an adherent dense coating or matrix of high purity silicon carbide ceramic at high yield.
The chlorine-free feature provides a noncorrosive reaction environment and product. In addition the silicon to carbon (Si:C) ratio is relatively low and can be controlled by control of parameters which affect the deposition rate. Carbon content of the coating can be varied from a slight excess of carbon to near stochiometric ratio of silicon to carbon using nitrogen as a carrier gas. No solvents or reactive secondary gases are required. While a carrier gas is not essential nitrogen, hydrogen, argon, and other suitable carriers can be used to vary flow rates and partial pressure of the TMTSH. Nitrogen, hydrogen, and mixtures thereof are useful for control of stochiometry of the coating.
Carbon content of the coating can be controlled to provide low friction products and articles having desirable thermal and electrical conductivity properties. Thermal decomposition of this silicon carbide precursor compound can be utilized as a practical method for making fine silicon carbide powder in a variety of sizes down to nano sizes.
The following chemical equations illustrate the reactions involved in making the silicon carbide precursor of this invention and will be more clearly understood when considered in conjunction with the examples of this disclosure:
Cl[CH3]2SiCH2Cl+0.3LiAlH4xe2x86x92H[CH3]2SiCH2ClH[CH3]2SiCH2Cl+Mg+THF0.5[CH3]Cl2SiHxe2x86x92H[CH3]2SiCH2SiH[CH3]CH2Si[CH3]2H.
TMTSH is prepared from chloromethyl -dimethylchlorosilane by reduction with lithium aluminum hydride in a suitable solvent such as butyl diglyme (diethylene glycol dibutyl ether) tetrahydrofuran, and other cyclic ethers or acyclic ethers. The resulting chloromethyldimethylsilane is then reacted with magnesium in tetrahydrofuran to form the corresponding Grignard reagent followed by coupling with methyldichlorosilane. The Grignard intermediate need not be isolated. It can be used directly to couple with methyldichlorosilane. The final product, TMTSH, can be recovered by distillation at atmospheric pressure to remove tetrahydrofuran followed by collection at an overhead temperature of about 80xc2x0 to 85xc2x0 C. and less than about 20 mm Hg pressure.
Preparation of 2,4,6-trimethyl-2,4,6-trisila-heptane is illustrated in the following examples.